In various kinds of display devices including a fluorescent display device whose light emitting unit employs a phosphor, a luminous color of the phosphor needs to be diversified to make a display thereof more diverse and more effective. Phosphors including cadmium (Cd) are known as phosphors for emitting light whose emission wavelengths range from yellow to red. However, since cadmium (Cd) is an environmentally unfriendly material, many restrictions are tend to be imposed very often when cadmium (Cd) is employed industrially, so that the employment of cadmium (Cd) needs to be reduced.
Therefore, an SrTiO3:Pr phosphor is recently developed as an alternative material. However, when the SrTiO3:Pr phosphor is used in a fluorescent luminous tube, a life span thereof is not favorable, and a scope of its application is limited.
Therefore, to solve the above-described problems, a method for improving the life span by adding Bi to the phosphor (for example, see Japanese Laid-Open Application No. H8-283709) is proposed, and, based on an observation that the life span of the phosphor is deteriorated due to the fact that barium (Ba) emitted from a filament thereof is attached thereto, a method for preventing a surface of the phosphor from being reduced by coating a substantially entire surface of the phosphor with a thin film of tungsten (W) (for example, see Japanese Laid-Open Application No. 2004-238549) is also proposed.
However, the conventional methods described above are not so effective, and, even when these methods are employed, the life span of the SrTiO3:Pr phosphor highly demanded as a phosphor for emitting light whose emission wavelength ranges from yellow to red is not sufficiently improved to make it attractive for a practical application. To be specific, whereas, for example, a ZnO:Zn phosphor which is widely used because its emission wavelength ranges from blue to green and its life span is sufficiently long has a life span of 30 kh, the conventional SrTiO3:Pr phosphor has a life span as short as 1 to 5 kh, and the conventional methods described above cannot improve the life span of the conventional SrTiO3:Pr phosphor to a make it appealing for a practical application.